In general terms, isochronous data is data which is non-packetized and of indeterminate, potentially continuous duration. Increasing availability of multi-media computers and work stations has contributed to an increase in interest in the transfer of data from isochronous data sources and sinks. An isochronous data source is a device which outputs data in a continuous stream, usually at a substantially constant average data rate. Examples include video cameras, which output a substantially continuous stream of data representing images and associated sounds, and telephone output, which can be a substantially continuous output of voice data (either analog or digitized). An example of an isochronous data sink is a video monitor which can receive a substantially continuous stream of video data for display.
Related to isochronous sources and sinks is the concept of data transfer which can also be either isochronous or non-isochronous. One type of non-isochronous data transfer is a packet-type transfer. As shown in FIG. 1A, data can be transferred in a plurality of packets 12a, 12b which can be either constant-sized or variable-sized. Each packet includes a field of data 14a, 14b which may be preceded and/or followed by non-data information such as preamble information 16a, 16b housekeeping information such as data source information, data destination information, and the like 18a, 18b and a frame end marker 20a. As seen in FIG. 1A, because the fields provided for data 14a, 14b are not substantially continuous, the packetized scheme of FIG. 1A is not isochronous but "bursty" in nature. An example of packetized data transfer is the commonly-used ethernet system. Standard ethernet systems are described in IEEE Standard 802.3. One implementation of which, known as 10 Base T, is described in the draft nine supplement to IEEE standard 802.3, dated Nov. 15, 1989.
Another type of non-isochronous data transfer is a token ring system. In a token ring system, a node is permitted to transmit data only after receipt of an electronic "token." As depicted in FIG. 1B, a first station may transmit a token 22a which is received 24a by a second station whereupon the second station may begin transmission of data 26a. After a period of data transmission, the second station transmits the token 22b which is received by a third station 24b that can then begin its own transmission of data 26b. As seen in FIG. 1B, because data transmission is synchronized with the occurrence of an event (the arrival of a token), the token ring system is not an isochronous data transfer system. One commonly used token ring Network is described in IEEE standard 802.5.
In contrast, FIG. 1C schematically depicts isochronous data transfer. In isochronous data transfer, the data transfer or connection is initiated, such as by initiating a telephone conversation or beginning a video camera transmission 30. After the connection is initiated, transmission of the data, possibly accompanied by transmission of housekeeping information (such as destinations, audio or video timing, and the like) is provided substantially continuously for an indeterminate period, such as until termination of the connection 32. Although it may be that not every bit transferred represents a data bit (since "housekeeping" bits may be also transferred), the transfer of data is substantially continuous in the sense that there are no substantial periods during which no data bits are transferred. It is possible that the data being transferred is "Null" data such as silence during a telephone conversation or transfer of a blank video image. One type of isochronous data transfer is the Fiber Distributed Data Interface-II (FDDI-II) as described, for example, in FDDI-II Hybrid Multiplexer, Revision 2.4, dated Mar. 25, 1991.
Previous attempts to accommodate isochronous data on a data network have resulted in characteristics which are disadvantageous for at least some applications. In some previous isochronous devices, the bandwidth available for accommodating a given isochronous source or sink was at least partially dependent on the demand on the network (i.e. on the total amount of data from and to sources and sinks transmitting and receiving on the network). In this type of system, performance of an isochronous source or sink could perceptibly degrade as additional sources or sinks were added to the network, such as by increasing data transfer delay. Preferably, both isochronous and non-isochronous bandwidth is provided, with the isochronous bandwidth being fixed and insensitive to non-isochronous demand and the non-isochronous bandwidth being fixed and insensitive to isochronous demand.
Many types of isochronous data transfer systems fail to provide for inter-operability with data derived from non-isochronous sources or sinks. In this type of system a given link is unable to transfer data from both an isochronous source/sink and a non-isochronous source/sink in a concurrent fashion (i.e. both within a time frame sufficiently short that the transfer is effectively simultaneous such that the ability of data sinks to process the data and the user's perception of the data are not substantially impacted). In these systems, it is infeasible to provide a single node which is coupled to both isochronous and non-isochronous source/sinks (such as a multimedia PC having ethernet capabilities and a video camera).
Some previous isochronous systems provide little or no compatibility with previous networks so that extensive replacement of hardware or software becomes necessary. For example, in some schemes, it is necessary to replace the physical medium such as twisted pair media, or if existing in-place physical media are used, the performance is degraded, such as by a decrease in bandwidth for the type of communication formerly carried by the existing media.
Some isochronous systems require installation of new Media Access Controllers (MAC) or provision of new application software (such as local area network software). Some previous isochronous systems introduce an undesirable degree of delay or "jitter" (data discontinuities). Some types of isochronous systems are inflexible in the amount of bandwidth provided for isochronous data such that if the data rate of an isochronous source or sink is not precisely matched to the available bandwidth, the bandwidth will be either overwhelmed or substantially under-utilized by the isochronous traffic.